Mass spectrometry is a widely used qualitative and quantitative method for analyzing various sample types. Because mass spectrometry instrumentation is typically large, bulky, and very sensitive to changes in climate and power supply, mass spectrometry analyses are typically performed in the laboratory rather than the field. Recently, these instruments are being reduced in size for the purposes of bringing the instruments to the field. This allows analytical scientists to perform analysis actually at the sample site, which can alleviate sample transport and preparation, and thereby reduce errors in analysis.
Certain types of mass spectrometers include ion traps for detection and subsequent measurement of ions having various mass-to-charge ratios. FIG. 1 depicts a hyperbolic Paul ion trap 100, which was invented by Wolfgang Paul in 1953. Hyperbolic Paul ion trap 100 comprises a toroidal ring electrode 105 having a hyperbolic cross section, situated between two end-cap electrodes 110, 115, which are also of hyperbolic cross section. In FIG. 1, z0 indicates the center-to-end cap spacing, and r0 indicates the ring electrode radius.
FIG. 2 depicts a cylindrical ion trap (CIT) 200, which is a derivative of the hyperbolic Paul ion trap. CIT 200 comprises an annular or barrel-shaped cylindrical ring electrode 210 situated between two flat end-cap electrodes 212a, 212b. In FIG. 2, zb indicates the ring electrode half-height, and thus the spacing between ring and end caps is given by (z0–zb). The concept of simplifying the hyperbolic ion trap geometry by approximating the hyperbolic electrode surfaces with flat electrodes was described by Langmuir et al. in 1962.
Ions can be trapped within a CIT volume by the appropriate application of radio-frequency (RF) and direct current (DC) voltages to the electrodes. For example, one method for trapping ions is to apply the RF voltage to the ring electrode and to ground the end-cap electrodes. Ions created inside the boundaries of the electrodes or introduced into the boundaries of the electrodes from an external ion source can be trapped in the oscillating potential well created by the applied RF voltage. A detailed description of ion motion in hyperbolic ion traps and CITs is beyond the scope of this document, and can be found in a number of publications. In addition to ion storage, the CIT can be used as a mass analyzer in a number of modes of operations.
The amplitude of the RF voltage waveform used to trap ions is related to the mass of the ions that are trapped in the device. Therefore, in order to alter the stability of the ions in the trap, the amplitude of the RF waveform can be altered. The primary method of mass analysis in an ion trap is referred to as “mass selective instability” and utilizes a ramp in the amplitude of the applied RF waveform to cause instability in the ion trap in a mass selective way. The amplitude of the RF waveform after it has ramped up can be quite high, such as in the thousands of volts.
In order to achieve an RF voltage level in the thousands of volts, a mass spectrometer typically includes a step up transformer that is used to amplify the RF signal to be applied to the ring electrode of the ion trap. Such step up transformers are known to include primary and secondary coils wound in a cylindrical shape and functioning as inductors. The cylindrical shape of the coil windings provides the transformer with a suitably high peak output voltage. In order to maintain the cylindrical shape of the coil windings, the windings can be wound around a non-conductive, non-magnetic cylindrical core. Alternatively, the coil can be wound in the air in the shape of a cylindrical coil spring without any supporting central core. Amplification of the RF signal can also be achieved with a single coil. In this case, amplification results from the resonance nature of the capacitive load of the ion trap and the inductance of the coil.
One drawback of cylindrically wound coils is that although they provide a suitably high peak output voltage with which to amplify the RF signals, they occupy a large amount of space within the mass spectrometer. For example, a cylindrical transformer is typically at least three inches in diameter and six inches in height. Therefore, coils of this type make it difficult to reduce the size of a mass spectrometer sufficient to allow the device to be truly portable.
Another drawback is that in order to precisely tune the RF signals, the voltage typically must be tuned by inductance. Tuning inductors are typically of large size. Therefore, this further increases the size of the mass spectrometer, reducing its portability.
In addition, because of the importance of the application of an accurate voltage and waveform to the CIT, the instruments must be carefully calibrated. For example, instruments are typically calibrated by analyzing a known compound by known fragmentation patterns under known voltage ramping parameters. After fragmentation, the voltage of the mass analyzer is ramped at a certain rate over time. As this voltage increases mass fragments having known mass to charge ratios are detected at certain times. By correlating the timing of the fragments detected with the timing of the voltage ramp the mass analyzer can be calibrated. While this calibration technique has been the standard for many years in the laboratory where the environment and power supplies are generally constant, for portable instruments reliance on this technique leads to erroneous determinations and necessitates multiple calibrations over time.
FIG. 3 illustrates an exemplary standard voltage ramp of the type typically provided to a CIT during mass analysis in prior art devices. FIG. 4 illustrates the mass spectra (relative abundance of ions versus the mass to charge ratio of the ions) of methyl salicylate under the ramping conditions of FIG. 3. FIG. 5 represents desired mass spectra of methyl salicylate. As shown in FIG. 3, the voltage tends to lose its linear function and level off over time. This level off is not desirable because it results in an inconsistent peak shapes in the resulting mass spectra. Ideally, the ramp would be perfectly linear and reproducible, which would result in constant peak shapes. For example, when the peak widths of the spectra of FIG. 4 are compared with the peak widths of the spectra of FIG. 5, it is apparent that the widths of FIG. 4 are less than desirable due to at least the peak broadening at higher mass to charge (“m/z”) ratios. This inconsistent peak width is a symptom of mis-calibration, which leads to erroneous analytical results.
Therefore, there is a need for a transformer of a mass spectrometer that can produce RF signals of suitably high voltage levels within a minimum of occupied space. There is also a need for improved calibration techniques for mass spectrometers.